Selective Oxygenation of Ring-Substituted Toluenes with Electron-Donating and -Withdrawing Substituents by Molecular Oxygen via Photoinduced Electron Transfer

Article abstract of DOI:10.1021/ja036645r

A ring-substituted toluene with an electron-withdrawing substituent, p-tolunitrile, is oxygenated by molecular oxygen to yield the corresponding aldehyde with tetrafluoro-p-dicyanobenzene as a photocatalyst under photoirradiation with an Hg lamp (λ > 300 nm). The oxygenation of a ring-substituted toluene with an electron-donating substituent, p-xylene, by molecular oxygen is also achieved with 10-methyl-9-phenylacridinium ion as a photocatalyst under visible light irradiation, yielding p-tolualdehyde exclusively as the final oxygenated product. Both the oxygenation reactions are initiated by photoinduced electron transfer from the ring-substituted toluene to the singlet excited state of the photocatalyst. The reason for the high selectivity in the photocatalytic oxygenation of various toluene derivatives by molecular oxygen is discussed on the basis of the photoinduced electron transfer mechanism that does not involve the autoxidation process (radical chain reactions). The reactive intermediates in the photocatalytic cycle are successfully detected as the transient absorption spectra and the electron spin resonance spectra.

Full text of DOI:10.1021/ja036645r

Published on Web 10/01/2003
Selective Oxygenation of Ring-Substituted Toluenes with
Electron-Donating and -Withdrawing Substituents by
Molecular Oxygen via Photoinduced Electron Transfer
†
†
‡
,†
Kei Ohkubo, Kyou Suga, Kohei Morikawa, and Shunichi Fukuzumi*
Contribution from the Department of Material and Life Science, Graduate School of
Engineering, Osaka UniVersity, CREST, Japan Science and Technology Corporation, Suita,
Osaka 565-0871, Japan, and Showa Denko, K. K., Kawasaki, Kanagawa 210-0867, Japan
Received June 12, 2003; E-mail: fukuzumi@ap.chem.eng.osaka-u.ac.jp.
Abstract: A ring-substituted toluene with an electron-withdrawing substituent, p-tolunitrile, is oxygenated
by molecular oxygen to yield the corresponding aldehyde with tetrafluoro-p-dicyanobenzene as a
photocatalyst under photoirradiation with an Hg lamp (λ > 300 nm). The oxygenation of a ring-substituted
toluene with an electron-donating substituent, p-xylene, by molecular oxygen is also achieved with 10-
methyl-9-phenylacridinium ion as a photocatalyst under visible light irradiation, yielding p-tolualdehyde
exclusively as the final oxygenated product. Both the oxygenation reactions are initiated by photoinduced
electron transfer from the ring-substituted toluene to the singlet excited state of the photocatalyst. The
reason for the high selectivity in the photocatalytic oxygenation of various toluene derivatives by molecular
oxygen is discussed on the basis of the photoinduced electron transfer mechanism that does not involve
the autoxidation process (radical chain reactions). The reactive intermediates in the photocatalytic cycle
are successfully detected as the transient absorption spectra and the electron spin resonance spectra.
Introduction
selective oxygenation of ring-substituted toluenes to aromatic
aldehydes has merited special attention because of useful
Molecular oxygen is an ideal reagent for economical and
environmental benign oxygenation reactions because of its
abundant availability and nontoxicity.1,2 Since direct concerted
applications of aromatic aldehydes as key chemical intermediates
for production of a variety of fine or specialty chemicals such
as pharmaceutical drugs, dyestuffs, pesticides, and perfume
3
reactions between singlet molecules and triplet oxygen ( O2
7
compositions. A number of methods using inorganic oxidants
3
-
Σg ) are spin-forbidden, activation of oxygen by transition
8
9
10
such as chromium(IV), cobalt(III), manganese(III), cerium-
3
metal catalysts, photoexcitation to produce singlet oxygen
11
12
(
IV), benzeneseleninic anhydride, or peroxydisulfate/copper
1
4,5
3
(
O2), or generation of radical species that can react with O2
1
3
ion have so far been reported for oxygenation of ring-
substituted toluenes to aromatic aldehydes. However, their
synthetic utility has been limited because of low yield and poor
selectivity. Moreover, the use of stoichiometric amounts of
inorganic oxidants should be avoided because of the environ-
mental problem. p-Tolunitrile, which has an electron-withdraw-
ing substituent (CN), is difficult to oxidize even by using
6
directly (spin-allowed) is required to use molecular oxygen as
a terminal oxidant. Among a variety of oxygenation reactions,
†
Osaka University.
Showa Denko.
‡
(
1) (a) Sheldon, R. A. J. Chem. Technol. Biotechnol. 1997, 68, 381. (b) Sheldon,
R. A. Chem. Ind. (London) 1997, 12. (c) Oxygenases and Model Systems;
Funabiki, T., Ed.; Kluwer: Dordrecht, The Netherlands, 1997.
(
2) (a) Barton, D. H. R.; Martell, A. E.; Sawyer, D. T. The ActiVation of
Dioxygen and Homogeneous Catalytic Oxidation; Plenum Press: New
York, 1993. (b) Simandi, L. L. Dioxygen ActiVation and Homogeneous
Catalytic Oxidation; Elsevier: Amsterdam, 1991.
14
inorganic oxidants to obtain p-cyanobenzaldehyde. p-Cy-
anobenzaldehyde has been synthesized industrially from p-
(
3) (a) Mukaiyama, T. Aldrich Acta 1996, 29, 59. (b) Sheldon, R. A.; Arends,
I. W. C. E.; Dijksman, A. Catal. Today 2000, 57, 157. (c) Tsuji, J. Synthesis
(7) (a) Franz, G.; Sheldon, R. A. Ullmann’s Encyclopedia of Industrial
Chemistry, 5th ed.; VCH: Weinheim, Germany, 1991. (b) Sheldon, R. A.;
Kochi, J. K. Metal Catalyzed Oxidation of Organic Compounds; Academic
Press: New York, 1981; Chapter 10.
(8) (a) Clarke, R.; Kuhn, A.; Okoh, E. Chem. Br. 1975, 11, 59. (b) Periasamy,
M.; Bhatt, M. V. Tetrahedron Lett. 1978, 4561.
(9) Ballard, R. E.; McKillop, A. U.S. Patent 4 482 438, 1984.
(10) Udupa, H. V. K. Trans. Soc. AdV. Electrochem. Sci. Technol. 1976, 11,
143.
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Lett. 1985, 26, 3353. (b) Ho, T.-L. Synthesis 1973, 347. (c) Syper, L.
Tetrahedron Lett. 1966, 4493.
(12) Barton, D. H. R.; Hui, R. A. H. F.; Lester, D. J.; Ley, S. V. Tetrahedron
Lett. 1979, 3331.
1
984, 369. (d) Barton, D. H. R. Tetrahedron 1998, 54, 5805.
(
4) Foote, C. S.; Clennan, E. L. Properties and Reactions of Singlet Oxygen.
In ActiVe Oxygen in Chemistry; Foote, C. S., Valentine, J. S., Greenberg,
A., Liebman, J. F., Eds.; Chapman and Hall: New York, 1995; pp 105-
1
40.
(
5) (a) Bellus, D. AdV. Photochem. 1979, 11, 105. (b) Gorman, A. A. AdV.
Photochem. 1992, 17, 217. (c) Lissi, E. A.; Encinas, M. V.; Lemp, E.;
Rubio, M. A. Chem. ReV. 1993, 93, 699. (d) Adam, W.; Prein, M. Acc.
Chem. Res. 1996, 29, 275.
(
6) (a) Ishii, Y.; Nakayama, K.; Takeno, M.; Sakaguchi, S.; Iwahama, T.;
Nishiyama, Y. J. Org. Chem. 1995, 60, 3934. (b) Ishii, Y.; Iwahama, T.;
Sakaguchi, S.; Nakayama, K.; Nishiyama, Y. J. Org. Chem. 1996, 61, 4520.
(c) Yoshino, Y.; Hayashi, Y.; Iwahama, T.; Sakaguchi, S.; Ishii, Y. J. Org.
Chem. 1997, 62, 6810. (d) Ishii, Y.; Sakaguchi, S.; Iwahama, T. J. Synth.
Org. Chem. Jpn. 1999, 57, 24. (e) Tashiro, Y.; Iwahama, T.; Sakaguchi,
S.; Ishii, Y. AdV. Synth. Catal. 2001, 343, 220. (f) Ishii, Y.; Sakaguchi, S.;
Iwahara, T. AdV. Synth. Catal. 2001, 343, 393.
(13) Bhatt, M. V.; Perumal, P. T. Tetrahedron Lett. 1981, 22, 2605.
(14) (a) Nicolaou, K. C.; Baran, P. S.; Zhong, Y.-L. J. Am. Chem. Soc. 2001,
123, 3183. (b) Nicolaou, K. C.; Montagnon, T.; Baran, P. S.; Zhong, Y.-L.
J. Am. Chem. Soc. 2002, 124, 2245.
12850
9
J. AM. CHEM. SOC. 2003, 125, 12850-12859
10.1021/ja036645r CCC: $25.00 © 2003 American Chemical Society

Selective Oxygenation of Ring-Substituted Toluenes
A R T I C L E S
cyanobenzoyl chloride,15 p-cyanobenzyl halide,16 or dichloro-
methylbenzonitrile.17
well-clarified by the detection of reactive intermediates in the
photocatalytic oxygenation reaction with use of laser flash
photolysis and electron spin resonance (ESR) measurements as
well as the analyses of the quantum yield determination.
Radical species that can react with molecular oxygen can be
18-20
readily generated by photoinduced electron-transfer reactions.
The radical cations of ring-substituted toluenes readily depro-
tonate to give the corresponding benzyl radicals, which can react
with oxygen directly to produce benzyl peroxyl radicals, leading
to the final oxygenated products.21 Rates of intermolecular
photoinduced electron transfer reactions normally increase with
increasing driving force of electron transfer (i.e., increasingly
Experimental Section
Materials. 1,4-Dicyanobenzene, 1,2,4,5-tetracyanobenzene, and
halogenated dicyanobenzenes were purchased commercially. Toluene,
p-, m-, and o-xylenes, 2,5-dimethoxytoluene, p-, m-, and o-tolualde-
hydes, p-tolunitrile, and p-tolualdehyde were also obtained com-
mercially. 10-Methylacridinium iodide (AcrH I ) was prepared by the
reaction of acridine with methyl iodide in acetone, converted to the
negative free energy change of electron transfer) to reach a
+ -
diffusion-limited value.2
2-25
The free energy change of electron
+
-
transfer is determined by the difference between the one-electron
oxidation potentials of electron donors and the one-electron
reduction potentials of electron acceptors. When the driving
force of electron transfer is small, the rate is highly sensitive to
perchlorate salt (AcrH ClO ) by the addition of magnesium perchlorate
4
+
-
to AcrH I in ethanol, and purified by recrystallization from metha-
nol.2 Likewise, 1-methyl-3-cyanoquinolinium, 1-methylquinolinium,
8,29
1,4-dimethylquinolinium, and 1,2-dimethylquinolinium perchlorate salts
22-25
were prepared by the reaction of the corresponding quinoline derivatives
with methyl iodide in acetone, followed by the metathesis with Mg-
minor changes in the driving force of electron transfer.
For
instance, only a 0.1 eV change in the driving force causes a 49
times difference in the electron-transfer rate.26 Thus, the
photoinduced electron transfer rates are finely controlled by
choosing photocatalysts with different one-electron redox
potentials. Since the one-electron oxidation potential of the
oxygenated product is significantly more difficult (i.e., shifted
to a more positive oxidation potential) than the reactant (electron
donor), an appropriate choice of substrate and photocatalyst
would enable selective oxygenation of the substrate via pho-
toinduced electron transfer from the substrate electron donor
to the excited state of the photocatalyst.
28,29
+
-
(
ClO
4
)
2
.
10-Methyl-9-phenylacridinium perchlorate salt (AcrPh ClO )
4
was prepared by the reaction of 10-methylacridone with the phenyl-
magnesium bromide in dichloromethane. Potassium ferrioxalate used
as an actinometer was prepared according to the literature and purified
by recrystallization from hot water. Acetonitrile (MeCN), dichlo-
30
31
romethane (CH Cl ), and chloroform (CHCl ) used as solvents were
2
2
3
32
2
purified and dried by standard procedures. Deuterated [ H
3
]acetonitrile
) were obtained from
Euri SO-TOP, CEA, France, and used as received.
Reaction Procedure. Typically, an MeCN solution (0.6 cm )
2
3 1 3
(CD CN) and deuterated [ H ]chloroform (CDCl
3
-
2
containing tetrafluoro-p-dicyanobenzene (1.0 × 10 M) and p-
-2
tolunitrile (3.0 × 10 M) in an NMR tube sealed with a rubber septum
was saturated with oxygen by bubbling with oxygen through a stainless
steel needle for 5 min. The solution was then irradiated with a mercury
lamp through an acetophenone-methanol filter transmitting λ > 300
nm at room temperature. The irradiated solution was analyzed periodi-
We report herein selective photoinduced oxygenation of ring-
substituted toluenes with electron-donating or -withdrawing
substituents by molecular oxygen that is achieved by choosing
appropriate photocatalysts for efficient photoinduced electron
transfer from ring-substituted toluenes to the excited state of
1
1
cally by H NMR spectroscopy. The H NMR measurements were
performed on a Japan Electron Optics JMN-AL300 (300 MHz) NMR
spectrometer. The products of the photooxygenation of toluenes (3.0
2
7
sensitizers. In contrast to oxidation by inorganic oxidants, no
further oxidation of the initial oxygenated product occurs via
photoinduced electron transfer to the sensitizer, leading to
formation of the initial oxygenated product as the sole oxygen-
ated product. The photoinduced electron-transfer mechanism of
the photocatalytic oxygenation of ring-substituted toluenes is
-2
+
-
-2
×
10 M) with AcrPh ClO
4
(1.0 × 10 M) in oxygen-saturated
3
1
1
CDCl (0.6 cm ) were determined by H NMR spectra. H NMR (300
3
MHz, CDCl ): p-tolualdehyde, δ 2.43 (s, 3H), 7.3-7.8 (m, 4H), 9.98
3
(s, 1H); m-tolualdehyde, δ 2.39 (s, 3H), 7.3-7.7 (m, 4H), 9.95 (s, 1H);
o-tolualdehyde, δ 2.64 (s, 3H), 7.2-7.8 (m, 4H), 10.24 (s, 1H);
benzaldehyde, δ 7.5-7.9 (m, 5H), 10.00 (s, 1H); isophthalaldehyde, δ
(
(
(
(
15) Rapoport, H.; Williams, A. R.; Lowe, O. G.; Spooncer, W. W. J. Am. Chem.
Soc. 1953, 75, 1125.
7
.8-8.4 (m, 4H), 10.13 (s, 2H); phthalaldehyde, δ 7.8-8.0 (m, 4H),
16) (a) Hass, H. B.; Bender, M. L. J. Am. Chem. Soc. 1949, 71, 1767. (b)
Wada, M.; Nakaoka, K. Japan Patent 60-166655, 1985.
10.53 (s, 2H). The product of the photooxygenation of p-tolunitrile
17) Sugiyama, M.; Nakagawa, M.; Matsuzawa, M. Japan Patent 09-227490,
-
2
-2
(
3.0 × 10 M) with tetrafluoro-p-dicyanobenzene (1.0 × 10 M) in
3 1
3
1
997.
18) (a) M u¨ ller, F.; Mattay, J. Chem. ReV. 1993, 93, 99. (b) Mattay, J.; Martin,
V. Top. Curr. Chem. 1991, 159, 219. (c) Photoinduced Electron Transfer;
Fox, M. A., Chanon, M., Ed.; Elsevier: Amsterdam, 1988; Part C.
19) (a) Julliard, M.; Chanon, M. Chem. ReV. 1983, 83, 425. (b) Julliard, M.;
Legris, C.; Chanon, M. J. Photochem. Photobiol. A: Chem. 1991, 61, 137.
oxygen-saturated CD
spectra. H NMR (300 MHz, CD
CN (0.6 cm ) were determined by H NMR
CN): p-cyanobenzaldehyde, δ 7.8-
1
3
(
8.1 (m, 4H), 10.05 (s, 1H); p-cyanobenzyl alcohol, δ 4.44 (s, 2H).
Quantum Yield Determination. A standard actinometer (potassium
(
b) Julliard, M.; Galadi, A.; Chanon, M. J. Photochem. Photobiol. A: Chem.
31
1
990, 54, 79. (c) Lopez, L. Top. Curr. Chem. 1990, 156, 117. (d) Heumann,
ferrioxalate) was used for the quantum yield determination of the
A.; Chanon, M. Applied Homogeneous Catalysis with Organometallic
Compounds; Cornils, B., Herrmann, W. A., Ed.; VCH: Weinheim,
Germany, 1996; pp 929-948.
+
AcrH -photosensitized oxygenation of a ring-substituted toluene with
oxygen. A square quartz cuvette (10 mm i.d.) that contained a CHCl
3
3
+
-
-4
(
(
(
(
(
(
(
(
20) Todd, W. P.; Dinnocenzo, J. P.; Farid, S.; Goodman, J. L.; Gould, I. R. J.
solution (3.0 cm ) of AcrH ClO
(1.0 × 10 M) and p-xylene [(5.0
4
Am. Chem. Soc. 1991, 113, 3601.
-2
×
10 )-1.0 M] was irradiated with monochromatized light of λ )
21) Fujita, M.; Ishida, A.; Takamuku, S.; Fukuzumi, S. J. Am. Chem. Soc. 1996,
1
18, 8566.
358 nm from a Shimadzu RF-5300PC fluorescence spectrophotometer.
22) (a) Rehm, A.; Weller, A. Ber. Bunsen-Ges. Phys. Chem. 1969, 73, 834.
(b) Rehm, A.; Weller, A. Isr. J. Chem. 1970, 8, 259.
23) Fukuzumi, S.; Ohkubo, K.; Suenobu, T.; Kato, K.; Fujitsuka, M.; Ito, O.
J. Am. Chem. Soc. 2001, 123, 8459.
(28) Roberts, R. M. G.; Ostovi c´ , D.; Kreevoy, M. M. Faraday Discuss. Chem.
Soc. 1982, 74, 257.
24) (a) Marcus, R. A. Annu. ReV. Phys. Chem. 1964, 15, 155. (b) Marcus, R.
(29) Fukuzumi, S.; Koumitsu, S.; Hironaka, K.; Tanaka, T. J. Am. Chem. Soc.
1987, 109, 305.
A. Angew. Chem., Int. Ed. Engl. 1993, 32, 1111.
25) Eberson, L. Electron-Transfer Reactions in Organic Chemistry; Springer-
(30) Fukuzumi, S.; Ohkubo, K.; Tokuda, Y.; Suenobu, T. J. Am. Chem. Soc.
2000, 122, 4286.
Verlag: Berlin, 1987.
26) The value is obtained from 0.1/(2.3k
B
T) ) (1.602 × 10- J)/[2.3(1.381 ×
19
(31) Hatchard, C. G.; Parker, C. A. Proc. R. Soc. London, Ser. A 1956, 235,
518.
-23
-1
1.69
1
0
J K )(298 K)] ) 1.69; 10
) 49.
27) A preliminary report has appeared: see Ohkubo, K.; Fukuzumi, S. Org.
Lett. 2000, 2, 3647.
(32) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of Laboratory
Chemicals, 4th ed.; Pergamon Press: Elmsford, NY, 1996.
J. AM. CHEM. SOC.
9
VOL. 125, NO. 42, 2003 12851

A R T I C L E S
Ohkubo et al.
Under the conditions of the actinometry experiments, both the acti-
Laser Flash Photolysis. The measurements of transient absorption
spectra in the AcrPh -photosensitized oxygenation of p-xylene with
+
-
+
nometer and AcrH ClO
4
absorbed essentially all the incident light.
The light intensity of monochromatized light of λ ) 358 nm was
oxygen were performed according to the following procedures. The
-
8
-1
determined as 2.78 × 10 einstein s with a slit width of 20 nm. The
photochemical reaction was monitored on a Shimadzu UV-3100PC
spectrophotometer by using diluted the reaction solution. The quantum
yields were determined from an increase in absorbance due to
2 3
argon-, air-, or O -saturated MeCN or CHCl solution containing
-
1
+
-4
p-xylene (1.0 × 10 M) and AcrPh (1.0 × 10 M) was excited by
a Nd:YAG laser (Continuum, SLII-10, 4-6 ns fwhm) at λ ) 355 nm
with the power of 30 mJ/pulse. Photoinduced events were estimated
by use of a continuous Xe lamp (150 W) and an InGaAs-PIN
photodiode (Hamamatsu 2949) as a probe light and a detector,
respectively. The output from the photodiodes and a photomultiplier
tube was recorded with a digitizing oscilloscope (Tektronix, TDS3032,
3
-1
-1
p-tolualdehyde (λ ) 290 nm, ꢀ ) 1.4 × 10
M
2
cm ) and
p-cyanobenzaldehyde (λ ) 309 nm, ꢀ ) 2.7 × 10 M cm-1). To
avoid the contribution by light absorption of the products, only the
initial rates were used for determination of the quantum yields.
-1
3
00 MHz). The transient spectra were recorded for fresh solutions in
Fluorescence Quenching. Quenching experiments of the fluores-
cence of photocatalysts by ring-substituted toluenes were performed
on a Shimadzu RF-5300PC fluorescence spectrophotometer. The
monitoring wavelength was that corresponding to the maximum of the
emission band of the photocatalyst. The solutions were deoxygenated
by argon purging for 10 min prior to the measurements. Relative
each laser excitation. All experiments were performed at 298 K.
+
Electron-Transfer Reduction of AcrPh . 10-Methyl-9-phenyl-
•
acridinyl radical (AcrPh ) in CHCl
electron-transfer reduction of AcrPh ClO
tetramethylsemiquinone radical anion (0-1.7 × 10 M) generated by
proportionation of tetramethyl-p-benzoquinone and tetramethylhydro-
quinone with tetrabutylammonium hydroxide.2
3
or MeCN was prepared by the
+
-
-4
4
(1.7 × 10 M) with
-
4
3
emission intensities were measured for MeCN or CHCl solutions
3,37
+
-6
containing a photocatalyst (e.g., AcrPh , 5.0 × 10 M) with electron
-
2
ESR Measurements. An oxygen- or argon-saturated dichlo-
donors at various concentrations (0-8.0 × 10 M). There was no
change in the shape but there was a change in the intensity of the
fluorescence spectrum by the addition of an electron donor. The Stern-
Volmer relationship
+
-2
romethane solution of AcrPh (1.0 × 10 M) and p-xylene (5.0 ×
M) was irradiated at 203 K with a high-pressure mercury lamp
USH-1005D) through a UV cutoff filter (λ < 310 nm) focused at the
-
1
1
0
(
sample cell in the ESR cavity. The ESR spectra were taken on a Jeol
JES-RE1XE and were recorded under nonsaturating microwave power
conditions. The magnitude of the modulation was chosen to optimize
the resolution and the signal-to-noise ratio (S/N) of the observed spectra.
The g values were calibrated by use of an Mn2 marker.
I0/I ) 1 + KSV[D]
(1)
was obtained for the ratio of the emission intensities in the absence
and presence of ring-substituted toluenes (I /I) and the concentrations
of quenchers [D]. The fluorescence lifetimes (τ) are 37 and 30 ns for
+
0
Results and Discussion
+
+
3
AcrH and 1.5 and 2.2 ns for AcrPh in MeCN and CHCl ,
Photoinduced Electron-Transfer Reactivities of Photo-
sensitizers. First, photoinduced electron-transfer reactivities of
a variety of photosensitizers were examined with p-tolunitrile
as an electron donor, which is difficult to oxidize because of
the electron-withdrawing substituent (CN), to find an appropriate
photosensitizer. The investigated photosensitizers are fluoro-,
chloro-, and cyano-substituted benzenes, 1-methylquinolinium,
and 10-methylacridinium ions as shown in Chart 1.
The redox and photophysical properties of the photosensitizers
were determined by electrochemical and photophysical mea-
surements (see Experimental Section), and data are summarized
in Table 1.
Irradiation of the absorption band of the photosensitizers
results in fluorescence in MeCN. The fluorescence of the
photosensitizers was quenched by p-tolunitrile, and the quench-
ing rate constants (kq) were determined from the slopes of the
Stern-Volmer plots and lifetimes of the singlet excited state
of the photosensitizers (typical data are shown in Supporting
2
1,23,33
respectively.
The lifetimes in the presence of electron donors were
determined by single photon counting on a Horiba NAES-1100 time-
resolved spectrofluorophotometer. The fluorescence lifetimes of halo-
genated dicyanobenzenes were measured by a Photon Technology
International GL-3300 with a Photon Technology International GL-
02, nitrogen laser/pumped dye laser system, equipped with a four-
channel digital delay/pulse generator (Stanford Research System Inc.
DG535) and a motor driver (Photon Technology International MD-
3
5
020). The excitation wavelength was 337 nm. The observed quenching
-
1
rate constants k () KSVτ ) were obtained from the Stern-Volmer
q
constants KSV and the emission lifetimes τ.
Electrochemical Measurements. Cyclic voltammetry (CV) mea-
surements were performed at 298 K on a BAS 100 W electrochemical
4 4
analyzer in deaerated MeCN containing 0.1 M Bu NClO (TBAP) as
supporting electrolyte. A conventional three-electrode cell was used
with a platinum working electrode (surface area of 0.3 mm ) and a
platinum wire as the counter electrode. The Pt working electrode (BAS)
was routinely polished with a BAS polishing alumina suspension and
rinsed with acetone before use. The second harmonic ac voltammetry
2
3
4,35
(
SHACV) measurements
a BAS 100B electrochemical analyzer. The measured potentials were
recorded with respect to the Ag/AgNO (0.01 M) reference electrode.
The redox potentials (vs Ag/AgNO ) are converted to those vs SCE
of toluene derivatives were performed on
Information, Figure S1). The k values thus obtained are also
q
summarized in Table 1.
The free energy change of photoinduced electron transfer
3
3
from p-tolunitrile to the singlet excited states of photosensitizers
3
6
by adding 0.29 V. All electrochemical measurements were carried out
under an atmospheric pressure of argon.
0
38
(
∆G et in electronvolts) is given by
0
0
0
*
red
∆G
) e(E - E
)
(2)
(
33) van Willigen, H.; Jones, G., II; Farahat, M. S. J. Phys. Chem. 1996, 100,
312.
et
ox
3
(34) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, Fundamental and
Applications; John Wiley & Sons: New York, 2001; Chapt. 10, pp 368-
0
0
*
where e is elementary charge and E ox and E red are the one-
electron oxidation potential of p-tolunitrile and the one-electron
4
16.
(
35) The SHACV method provides a superior approach to directly evaluating
the one-electron redox potentials in the presence of a follow-up chemical
reaction, relative to the better-known dc and fundamental harmonic ac
methods: (a) Bond, A. M.; Smith, D. E. Anal. Chem. 1974, 46, 1946. (b)
Arnett, E. M.; Amarnath, K.; Harvey, N. G.; Cheng, J.-P. J. Am. Chem.
Soc. 1990, 112, 344.
(37) (a) Fukuzumi, S.; Nakanishi, I.; Maruta, J.; Yorisue, T.; Suenobu, T.; Itoh,
S.; Arakawa, R.; Kadish, K. M. J. Am. Chem. Soc. 1998, 120, 6673. (b)
Fukuzumi, S.; Nakanishi, I.; Suenobu, T.; Kadish, K. M. J. Am. Chem.
Soc. 1999, 121, 3468.
(38) Kavarnos, G. J. Fundamentals of Photoinduced Electron Transfer; Wiley-
VCH: New York, 1993.
(
36) Mann, C. K.; Barnes, K. K. Electrochemical Reactions in Nonaqueous
Systems; Mercel Dekker: New York, 1970.
12852 J. AM. CHEM. SOC.
9
VOL. 125, NO. 42, 2003

Selective Oxygenation of Ring-Substituted Toluenes
A R T I C L E S
Chart 1
containing p-tolunitrile (30 mM) with an Hg lamp results in
formation of p-cyanobenzaldehyde accompanied by disappear-
ance of p-tolunitrile (eq 4). The product yield increase with
photoirradiation time is shown in Table 2.42,43
The quantum yields (Φ) of the tetrafluoro-p-dicyanobenzene-
photocatalyzed oxygenation of p-tolunitrile with O2 were
determined from the product formation rate under irradiation
of monochromatized light of λ ) 312 nm (see Experimental
Section). The same Φ values are obtained for the phoooxygen-
ation reaction at different concentrations of O2 (Figure 2a). The
Φ values increase with an increase in concentration of p-
tolunitrile [D] to approach a limiting value (Φ∞) in accordance
with eq 5
Φ ) Φ Kobs[D]/(1 + Kobs[D])
(5)
∞
reduction potential of the singlet excited state of photosensitizers,
respectively.3
Figure 1 shows a plot of log kq vs E red in MeCN, which
as shown in Figure 2a. Equation 5 is rewritten as eq 6
9,40
Φ- ) Φ∞-1[1 + (Kobs[D]) ]
1
-1
(6)
0
*
exhibits a typical feature of an electron-transfer process: the
-
1
-1
0
*
and the linear plots of Φ versus [D] is shown in Figure 2b.
The Φ∞ and Kobs values are obtained from the slope and intercept
log kq value increases with an increase in E red , which
0
corresponds to a decrease in ∆G et in eq 2, to reach a plateau
-
1
in Figure 2b as 0.47 and 14 M , respectively. The Kobs value
can be converted to the corresponding rate constant (kobs)
provided that the excited state of tetrafluoro-p-dicyanobenzene
involved in the photocatalytic reaction is a singlet (kobs )
value corresponding to the diffusion rate constant in MeCN (2.0
1
0
-1 -1
×
10
M
s ) as the photoinduced electron transfer becomes
2
2,38
energetically more favorable (i.e., more exergonic).
dependence of kq on ∆G et for adiabatic outer-sphere electron
The
0
-
1
9
-1 -1
Kobsτ , τ ) 3.2 ns). The kobs value (4.6 × 10 M s ) agrees
transfer has well been established by Marcus as given by
well with the corresponding kq value determined independently
9
-1 -1
1
1
1
by the fluorescence quenching (4.9 × 10 M s ; Supporting
Information, Figure S1). Such agreement strongly indicates that
the photocatalytic reaction proceeds via photoinduced electron
transfer from p-tolunitrile to the singlet excited state of
tetrafluoro-p-dicyanobenzene as shown in Scheme 1.
Electron transfer occurs from p-tolunitrile to the singlet
excited state of tetrafluoro-p-dicyanobenzene (ket) to give the
radical ion pair. Deprotonation of p-tolunitrile radical cation
(RH ) occurs in competition with the back electron transfer
from tetrafluoro-p-dicyanobenzene radical anion to p-tolunitrile
radical cation. Benzyl radical (R ), which is the deprotonation
product of p-tolunitrile radical cation, reacts with molecular
oxygen to give the peroxyl radical (Scheme 1). Protonation of
the peroxyl radical occurs to give the hydroperoxide, which
decomposes to p-cyanobenzaldehyde.
According to Scheme 1, the quantum yield Φ is given as a
function of concentrations of p-tolunitrile [D] by
)
+
(3)
0
et
2
kq kdiff
Z exp[-(λ/4)(1 + ∆G /λ) /k T]
B
1
0
where kdiff is the diffusion rate constant, taken as 2.0 × 10
-1
-1
M
s
in MeCN; Z is the collision frequency, taken as 1 ×
011
-1
-1
1
M
s ; λ is the reorganization energy of electron transfer;
24,38
and kB is the Boltzmann constant.
The solid line in Figure
1
calculated from eqs 2 and 3 with the λ value of 0.33 eV and
•
+
0
41
E ox value of 2.64 V agrees with the experimental kq values.
Thus, the fluorescence quenching of photosensitizers by p-
tolunitrile occurs via electron transfer from the p-tolunitrile to
the singlet excited states of photosensitizers.
•
Photocatalytic Oxygenation of p-Tolunitrile with Oxygen
by Use of Tetrafluoro-p-dicyanobenzene. Tetrafluoro-p-di-
cyanobenzene was chosen as a photocatalyst for oxygenation
of p-tolunitrile with molecular oxygen, since the photoinduced
electron transfer from p-tolunitrile to the singlet excited state
of tetrafluoro-p-dicyanobenzene occurs efficiently (Table 2).
Photoirradiation of the absorption band of tetrafluoro-p-dicy-
anobenzene (λmax ) 312 nm) in oxygen-saturated acetonitrile
Φ ) k k τ[D]/(k + k ) (1 + k τ[D])
(7)
d
et
d
bet
et
which agrees with the experimental observation (eq 5). The Φ
value is independent of concentrations of O2, since the limiting
(
39) Fukuzumi, S.; Tanaka, T. In Photoinduced Electron Transfer; Fox, M. A.,
Chanon, M., Eds.; Elsevier: Amsterdam, 1988; Part C, p 578.
40) (a) Fukuzumi, S.; Kitano, T. J. Chem. Soc., Perkin Trans. 2 1991, 41. (b)
Fukuzumi, S.; Noura, S. J. Chem. Soc., Chem. Commun. 1994, 287. (c)
Fukuzumi, S.; Fujita, M.; Noura, S.; Ohkubo, K.; Suenobu, T.; Araki, Y.;
Ito, O. J. Phys. Chem. A 2001, 105, 1857.
(
(42) At prolonged reaction times, p-cyanobenzyl alcohol is also produced,
probably due to the autoxidation process.
(43) In the case of 1,4-dicyanobenzene, photooxygenation of p-tolunitrile hardly
occurred under the same experimental conditions as employed for the case
of tetrafluoro-p-dicyanobenzene used as a photocatalyst. This is consistent
(
41) The E0ox value of p-tolunitrile in MeCN could not be determined directly
by the electrochemical measurement because of the high oxidation potential
and the irreversible oxidation.
with the much smaller k
q
value of 1,4-dicyanobenzene than that of
tetrafluoro-p-dicyanobenzene (Table 1).
J. AM. CHEM. SOC.
9
VOL. 125, NO. 42, 2003 12853

A R T I C L E S
Ohkubo et al.
Table 1. One-Electron Reduction Potentials, Fluorescence Lifetimes, Singlet Excited Energies, and Quenching Rate Constants in MeCN
E0red (V) vs SCE
a
E0red* b (V) vs SCE
τc (ns)
E
00(S) (eV)
d
e
q
k )
(M-1 s-1
photosensitizer
1
9
9
9
9
8
8
8
7
0
1
2
3
4
5
6
7
8
9
0
1
2
1,2,4,5-tetracyanobenzene
1-methyl-3-cyanoquinolinium ion
tetrafluoro-p-dicyanobenzene
tetrafluoro-o-dicyanobenzene
tetrafluoro-m-dicyanobenzene
1,4-dicyanobenzene
-0.74
3.17
2.72
2.66
2.66
2.61
2.55
4.3
45
3.81
3.32
3.76
4.18
3.99
4.01
1.5 × 10
7.3 × 10
4.9 × 10
3.4 × 10
3.3 × 10
7.4 × 10
2.2 × 10
1.2 × 10
8.7 × 10
h
f
f
f
f
-0.60
-1.10
-1.62
-1.33
-1.46
3.2
2.6
3.9
9.7
f
f
f
f
1-methylquinolinium ion
-0.96
2.54
2.51
20
19
15
37
3.50
3.58
f
f
f
f
1,4-dimethylquinolinium ion
1,2-dimethylquinolinium ion
10-methylacridinium ion
tetrachloro-p-dicyanobenzene
tetrachloro-m-dicyanobenzene
-1.07
f
f
f
f
-1.05
2.46
3.51
g
2.75g
3.50
3.71
1
1
1
-0.43g
-0.95
j
2.32g
2.55
0.25
0.20
i
i
a
One-electron reduction potential of ground state. One-electron reduction potential of singlet excited state. c Fluorescence lifetime of dicyanobenzene
b
d
e
derivatives. Singlet excited energy of dicyanobenzene derivatives. Quenching rate constants of the photoinduced electron transfer from p-tolunitrile to
photosensitizer in MeCN. Taken from ref 40. Taken from ref 23. h Too slow to be determined accurately. No quench. Irreversible CV wave.
f
g
i
j
0
*
Figure 1. Plots of log kq vs E red for the fluorescence quenching of various
photosensitizers by p-tolunitrile in deaerated MeCN at 298 K. The curves
0
represent the best fit to eqs 2 and 3; E ox ) 2.64 V vs SCE and λ ) 7.6
-
1
kcal mol . Numbers refer to photosensitizers in Table 1.
Table 2. Reactant Conversion and Product Yields in
-
2
Photooxygenation of p-Tolunitrile (3.0 × 10 M), Catalyzed by
-
2
Tetrafluoro-p-Dicyanobenzene (1.0 × 10 M) in O2-Saturated
MeCN
Figure 2. (a) Dependence of the quantum yield (Φ) of p-cyanobenzalde-
-
4
hyde for the tetrafluoro-p-dicyanobenzene-catalyzed (1.1 × 10 M)
photooxygenation of p-tolunitrile in oxygen- (b) and air- (O) saturated
-
1
-1
MeCN at 298 K. (b) Plots of Φ vs [p-tolunitrile]
.
quantum yield Φ∞ is determined by competition between the
deprotonation of the radical cation (kd) and the back electron
transfer (kbet): Φ∞ ) kd /(kd + kbet). It should be noted that no
radical chain process (autoxidation) is involved in Scheme 1.
Selective Photocatalytic Oxygenation of p-Xylene with
Oxygen by Use of 10-Methyl-9-phenylacridinium Perchlo-
rate. The oxygenation of a ring-substituted toluene with an
electron-donating substituent, p-xylene, by molecular oxygen
is also achieved by use of 10-methylacridinium perchlorate
+
-
(AcrH ClO ) as a photocatalyst. Visible light irradiation of
4
+
the absorption band (λmax ) 358 and 417 nm) of AcrH (10
mM) in oxygen-saturated acetonitrile containing p-xylene (30
12854 J. AM. CHEM. SOC.
9
VOL. 125, NO. 42, 2003

Selective Oxygenation of Ring-Substituted Toluenes
A R T I C L E S
Scheme 1
Table 4. Fluorescence Quenching Rate Constants of AcrH+ and
AcrPh by Ring-Substituted Toluenes and Aldehydes in Deaerated
+
MeCN at 298 K
a
Values in parentheses were determined in CHCl3. b Benzaldehyde.
Table 3. Photooxygenation Yields of Xylenes and Toluene (3.0 ×
o-, m-, and p-tolualdehyde and benzaldehyde after 10 h of
-
2
+
-2
10
M), Catalyzed by AcrPh (1.0 × 10 M) with O2 in
photoirradiation of an oxygen-saturated chloroform solution of
a
O2-Saturated Chloroform at 298 K
-
2
+
xylenes and toluene (3.0 × 10 M) containing AcrPh (1.0 ×
-
2
1
0
M) with a mercury lamp (λ < 300 nm) decreases in the
order p-xylene > o-, m-xylene > toluene. The selectivity for
tolualdehyde decreases in the order p- (100%) > m- (99%) >
o-xylene (94%). The further oxygenation of m- and o-xylene
occurs to yield small amounts of the corresponding phthalal-
dehyde.
Detection of Reaction Intermediates. The occurrence of
1
+*
photoinduced electron transfer from p-xylene to AcrPh is
confirmed by laser flash photolysis experiments (see Experi-
mental Section). Laser excitation (λ ) 355 nm from a Nd:YAG
+
-4
laser) of AcrPh (1.0 × 10 M) in deaerated acetonitrile
-1
solution containing p-xylene (1.0 × 10 M) affords a transient
a
Irradiation time is 10 h. b Benzaldehyde.
absorption spectrum at 1 µs with appearance of new absorption
+
bands at 330, 430, 520, and 700 nm with bleaching of AcrPh
mM) with a xenon lamp through a UV cutoff filter (λ < 310
nm) results in formation of p-tolualdehyde accompanied by
disappearance of p-xylene. The product was identified by the
at 360 and 420 nm as shown in Figure 3a. Similar transient
absorption spectra are also observed in CHCl3 (see Supporting
1
Information, Figure S2). The transient absorption band at λmax
H NMR spectrum (see Experimental Section). After 24 h of
•
)
520 nm is assigned to AcrPh , since the absorption spectrum
irradiation, the yield of p-tolualdehyde was 37%. The product
yield is improved to 66% when acetonitrile is replaced by a
less polar solvent, chloroform, under otherwise the same
experimental conditions. The photooxygenated product yield is
•
of AcrPh produced independently by the electron-transfer
reduction of AcrPh by tetramethylsemiquinone radical anion
+
(Figure 3b) agrees with the transient absorption spectrum (Figure
+
+
3a). The absorption band at 330 nm is assigned to p-xylenyl
radical, which may be produced by deprotonation of p-xylene
further improved to 100% when AcrH is replaced by AcrPh
in chloroform (eq 8) as shown in Table 3. There were no
4
4,45
radical cation.
The broad absorption band at λmax ) 700
nm may be assigned to p-xylene dimer radical cation, since
similar broad transient absorption bands at a long-wavelength
region have been reported for the radical cations of toluene and
4
6-49
other aromatic compounds.
dioxygenated products such as p-phthalaldehyde and p-toluic
(44) Izumida, T.; Ichikawa, T.; Yoshida, H. J. Phys. Chem. 1980, 84, 60.
(
45) (a) Sehested, K.; Holcman, J. J. Phys. Chem. 1978, 82, 651. (b) Hatanaka,
K.; Itoh, T.; Asahi, T.; Ichinose, N.; Kawanishi, S.; Sasuga, T.; Fukumura,
H.; Masuhara, H. J. Phys. Chem. A 1999, 103, 11257.
acid after prolonged photoirradiation. It was confirmed that there
+
was no adduct formation between the photocatalyst, AcrPh ,
(
46) Liu, A.; Sauer, M. C., Jr.; Loffro, D. M.; Trifunac, A. D. J. Photochem.
Photobiol. A: Chem. 1992, 67, 197.
and p-xylene. Thus, the 100% selective photooxygenation of
p-xylene to p-tolualdehyde has been accomplished by use of
(
47) (a) Kochi, J. K.; Rathore, R.; Magu e` res, P. L. J. Org. Chem. 2000, 65,
6
826. (b) Yokoi, H.; Hatta, A.; Ishiguro, K.; Sawaki, Y. J. Am. Chem.
+
AcrPh as a photocatalyst in chloroform. The photoirradiation
Soc. 1998, 120, 12728.
-
2
(48) (a) Bally, T.; Roth, K.; Straub, R. J. Am. Chem. Soc. 1988, 110, 1639. (b)
Badger, B.; Brocklehurst, B. Trans. Faraday Soc. 1969, 65, 2582 and 2588.
(c) Gerson, F.; Kaupp, G.; Ohya-Nishiguchi, O. Angew. Chem., Int. Ed.
Engl. 1977, 16, 657. (d) Badger, B.; Brocklehurst, B. Trans. Faraday Soc.
time to obtain 100% yield of p-tolualdehyde (3.0 × 10 M)
was reduced from 24 to 10 h when a xenon lamp was replaced
by a high-pressure mercury lamp (1000 W) through an aceto-
phenone-methanol filter (λ < 300 nm).
1
970, 66, 2939. (e) Rodgers, M. A. J. J. Chem. Soc., Faraday Trans. 1972,
68, 1278. (f) Inokuchi, Y.; Naitoh, Y.; Ohashi, K.; Saitow, K.-I.; Yoshihara,
K.; Nishi, N. Chem. Phys. Lett. 1997, 269, 298.
Other isomers, o- and m-xylene, are also converted to o- and
m-tolualdehyde, respectively (Table 4). The product yields of
(
49) For dimer radical anions of electron acceptors, see Ganesan, V.; Rosokha,
S. V.; Kochi, J. K. J. Am. Chem. Soc. 2003, 125, 2559.
J. AM. CHEM. SOC.
9
VOL. 125, NO. 42, 2003 12855

A R T I C L E S
Ohkubo et al.
Figure 4. (a) Plots of ∆Abs at 700 nm vs [p-xylene] in the photoinduced
+
Figure 3. (a) Transient absorption spectra observed by photoexcitation of
the argon- (O), air- (4), and oxygen- (b) saturated MeCN solution of
electron transfer from p-xylene to AcrPh in deaerated CHCl3 at 1.0 µs
-
1
-1
after laser excitation at 298 K. (b) Plots of ∆Abs vs [p-xylene]
.
+
-4
-1
AcrPh (1.0 × 10 M) and p-xylene (1.0 × 10 M) at 1 µs after laser
•
-
+
excitation at 298 K. (b) Spectral change upon addition of Me4Q (0, 2.8
When oxygen is introduced to the AcrPh -p-xylene system
in acetonitrile, the broad absorption band at 700 nm due to
p-xylene dimer radical cation disappears as shown in Figure 3a
-
5
-5
-5
-4
-4
-4
×
10 , 5.6 × 10 , 8.4 × 10 , 1.1 × 10 , 1.4 × 10 , and 1.7 × 10
+ -4
M) to a deaerated MeCN solution of AcrPh (1.7 × 10 M) at 298 K.
(
air- and oxygen-saturated solution), whereas the absorption
To confirm the generation of p-xylene dimer radical cation,
dependence of ∆Abs at 700 nm on the concentration of p-xylene
was examined in CHCl3 as shown in Figure 4a, where ∆Abs at
•
band due to AcrPh at 520 nm remains. This indicates that
oxygen reacts with p-xylenyl radical produced by deprotonation
of p-xylene radical cation, which is in equilibrium with the dimer
radical cation, rather than with AcrPh . The deprotonation of
radical cations of toluene and p-xylene is known to occur
7
00 nm increases with an increase in the concentration of
•
p-xylene to approach a limiting value. Such a saturated
dependence of ∆Abs on concentration of p-xylene indicates the
existence of an equilibrium between the monomer and the dimer
radical cation. The formation constant of the dimer radical cation
4
5,50
efficiently.
In the presence of oxygen, p-xylenyl radical
reacts with oxygen in competition with the back electron transfer
from AcrPh to p-xylene radical cation to give the peroxyl
radical, which was detected successfully by ESR (vide infra).
•
2
-1
is determined as 1.4 × 10 M from the slope and the intercept
-
1
-1
of a linear plot of ∆Abs vs [p-xylene] (Figure 4b).
An oxygen-saturated dichloromethane solution of p-xylene
•
The absorption bands at 520 nm due to AcrPh decays
obeying second-order kinetics. The second-order plot of [AcrPh ]
-
1
+
-2
(
5.0 × 10 M) with AcrPh (1.0 × 10 M) was irradiated by
•
a high-pressure mercury lamp at 203 K. The resulting ESR
spectrum consists of two isotropic signals at g ) 2.0151 and at
g ) 2.0034 (Figure 5a). The former signal is readily assigned
to p-xylenylperoxyl radical because the g value is diagnostic
obtained from the absorbance at 520 nm with the ꢀ value of
3
-1
-1
3
.9 × 10 M cm can be fit to a line (Supporting Information,
•
Figure S3). The ꢀ value of AcrPh in CHCl3 is determined from
the titration of AcrPh with tetramethylsemiquinone radical
+
5
1
•
of peroxyl radicals. The latter signal is assigned to AcrPh ,
anion (Figure S3). The second-order decay rate constant is
10
-1 -1
determined from the slope as 1.1 × 10
nearly the same as the diffusion-limited value in CHCl3 (1.2 ×
s ).
M
s , which is
(50) (a) Masnovi, J. M.; Kochi, J. K. J. Am. Chem. Soc. 1985, 107, 6781. (b)
Masnovi, J. M.; Kochi, J. K. J. Phys. Chem. 1987, 91, 1878. (c) Magu e` res,
P. L.; Lindeman, S. V.; Kochi, J. K. J. Chem. Soc., Perkin Trans. 2, 2001,
1180.
10
-1 -1 38
1
0
M
12856 J. AM. CHEM. SOC.
9
VOL. 125, NO. 42, 2003

Selective Oxygenation of Ring-Substituted Toluenes
A R T I C L E S
Figure 5. ESR spectra observed under photoirradiation of (a) oxygen- and
+
-2
(
b) argon-saturated CH2Cl2 solution of AcrPh (1.0 × 10 M) containing
-
1
2
+
p-xylene (5.0 × 10 M) at 203 K. The asterisk denotes an Mn ESR
marker.
Scheme 2
Figure 6. (a) Dependence of the quantum yield (Φ) of p-tolualdehyde for
+
-4
the AcrPh -catalyzed (1.1 × 10 M) photooxygenation of p-xylene in air-
-
1
(
O) and oxygen- (b) saturated CHCl3 at 298 K. (b) Plots of Φ vs
1
-
[
p-xylene]
.
cation as seen in Figure 3a. This is followed by deprotonation
of p-xylene radical cation to give p-xylenyl radical in competi-
tion with the back electron transfer (kbet) to the reactant pair. In
the presence of oxygen, p-xylenyl radical is readily trapped by
oxygen to give p-xylenylperoxyl radical as observed in Figure
5
a. The p-xylenylperoxyl radical is reduced by back electron
•
transfer from AcrR to yield p-xylenyl hydroperoxide, ac-
companied by regeneration of AcrR (Scheme 2). The hydro-
+
on the basis of the agreement between the reported and observed
g value.23 In the absence of oxygen, only the latter signal is
peroxide decomposes to yield p-tolualdehyde selectively.
According to Scheme 2, the quantum yield (Φ) for the AcrR -
+
observed under otherwise identical experimental conditions
Figure 5b).52
catalyzed photooxygenation of p-xylene is expressed as a
function of concentration of p-xylene as in the case of the
photocatalytic oxygenation of p-tolunitrile (eq 7). The Φ values
(
Photocatalytic Oxygenation Mechanism. Based on the
+
above results, the reaction mechanism for the AcrR -photo-
sensitized oxygenation (R ) H and Ph) of p-xylene is given as
shown in Scheme 2. Photoinduced electron transfer from p-
+
of the AcrPh -catalyzed photooxygenation of p-xylene with O2
in air- and oxygen-saturated CHCl3 were determined from the
product formation rate under irradiation of monochromatized
light of λ ) 358 nm (see Experimental Section). The Φ values
were the same at different oxygen concentrations (Figure 6a).
The Φ value increases with an increase in concentration of
p-xylene to approach a limiting value (Φ∞) in accordance with
1
+
•
xylene to AcrR * (ket) occurs to produce AcrR and p-xylene
radical cation, which is in equilibrium with the dimer radical
(
51) Cumylperoxyl radical has been reported as g ) 2.0156: (a) Bersohn, M.;
Thomas, J. R. J. Am. Chem. Soc. 1964, 86, 959. (b) Fukuzumi, S.; Ono, Y.
J. Chem. Soc., Perkin Trans. 2 1977, 622.
-1
-1
eq 7 (Figure 6a). The linear plot of Φ and [p-xylene] in
(
52) The ESR signal due to p-xylene dimer radical cation may be overlapped
•
with that of AcrPh .
accordance with eq 8 as shown in Figure 6b. From the slope
J. AM. CHEM. SOC.
9
VOL. 125, NO. 42, 2003 12857

A R T I C L E S
Ohkubo et al.
Scheme 3
in Table 4. Thus, the faster the photoinduced electron transfer,
the larger the product yield. However, the kq value for p-xylene
9
-1 -1
determined in chloroform (4.2 × 10 M s ) is smaller than
the value in acetonitrile in Table 4, in contrast to the improved
product yield in chloroform as compared to that in the more
polar solvent acetonitrile (vide supra).
The improved product yield in chloroform may result from
a decrease in the reorganization energy for the electron transfer
with decreasing the solvent polarity, which results in the slower
and intercept in Figure 6b, the Φ∞ and Kobs values are obtained
as 0.10 and 14 M , respectively. The Kobs value can be
-
1
converted to the corresponding rate constant (kobs) provided that
•
+
back electron transfer from AcrH to p-xylene radical cation in
the excited state of AcrPh involved in the photocatalytic
2
3
-
1
1
+*
Scheme 2 (vide infra). Since the deprotonation of p-xylene
radical cation, which leads to the oxygenated product, competes
with the back electron transfer, the slower back electron transfer
results in the larger product yield.
reaction is a singlet (kobs ) Kobsτ , τ ) 2.2 ns for AcrPh ).
9
-1 -1
The kobs value thus obtained (6.3 × 10 M s ) agrees well
9
-1 -1
with the corresponding ket value (6.9 × 10 M s ) determined
independently by fluorescence quenching. Such agreement
strongly supports the reaction mechanism in Scheme 2 where
the photocatalytic reaction is initiated by photoinduced electron
The smaller reorganization energy in CHCl3 (0.27 eV) than
in acetonitrile (0.34 eV) has been confirmed by determining
the rate constants of electron-transfer self-exchange reactions
1
+*
transfer from p-xylene to AcrR .
+
between 10-methyl-9-phenylacridinium ion (AcrPh ) and the
The thermal oxygenation reaction of p-xylene with oxygen
•
corresponding one-electron-reduced radical (AcrPh ) in aceto-
is proposed to proceed via radical chain processes as shown in
2
3
Scheme 3.53-55
nitrile and chloroform. Since the λ values (0.27, 0.34 eV) are
The p-xylene radical cation is assumed to be
much smaller than the driving force of the back electron transfer
produced by direct electron transfer from p-xylene to oxygen
in a charge-transfer complex between them.54 However, the
saturated dependence of Φ on [D] in Figure 6a indicates that
such an electron-transfer radical chain process (Scheme 3) is
not operative as the major pathway in the present photocatalytic
reaction.
0
•
0
29
(-∆G et ) 2.36 eV) from AcrH (E ox vs SCE ) -0.43 V)
0
23
to p-xylene radical cation (E red ) 1.93 V), the back electron
transfer is deeply in the Marcus inverted region, where the back
electron- transfer rate is expected to slow with decreasing λ
23,24
value.
The slower back electron-transfer rate with decreasing
solvent polarity leads to an increase in the product yield as
observed experimentally (see Supporting Information, Figure
S5).
If the chain process in Scheme 3 were the major pathway,
the Φ value would increase linearly with increasing concentra-
tion of p-xylene.
Further improvement of the product yield by employing
No photooxygenation of 2,5-dimethoxytoluene, the strongest
+
+
+
AcrPh instead of AcrH can also be ascribed to the slower
back electron-transfer rate for the former than the latter. Since
electron donor in this study, has occurred with AcrPh as a
photocatalyst in CHCl3, probably because of the slow depro-
tonation rate as compared with the fast back electron transfer
0
•
0
23
the E ox value of AcrPh (E ox vs SCE ) -0.55 V) is more
negative than the value of AcrH (E ox vs SCE ) -0.43 V),
the driving force of the back electron transfer from AcrPh (2.48
eV) is larger than that from AcrH (2.36 eV). The larger driving
•
0
29
+
despite the efficient fluorescence quenching of AcrPh by 2,5-
•
1
0
-1 -1
dimethoxytoluene (kq ) 1.2 × 10
M
s ).
•
The 100% selective photocatalytic oxygenation of p-xylene
is made possible by the difference in the reactivity of p-xylene
and the oxygenated product, p-tolualdehyde, as indicated by the
following fluorescence quenching experiments. The fluorescence
force thereby results in slower back electron transfer, leading
to improved product yield.
+
The enhanced stability of AcrPh as a photocatalyst as
+
+
compared to AcrH is ascribed to the steric effect of the phenyl
lifetimes (τ) of AcrH (λem ) 488 nm) in the absence and
•
group of AcrPh , which hampers the radical coupling with
presence of xylenes, toluene, or the corresponding aldehydes
were determined with the time-resolved fluorescence spectro-
fluorophotometer. The rate constants of fluorescence quenching
deprotonated radicals that could be the deactivation process of
the photocatalyst in Scheme 2.
In conclusion, the use of AcrPh+ as a photocatalyst in
chloroform has enabled us to accomplish the 100% selective
photooxygenation of p-xylene to p-tolualdehyde as well as
highly selective photooxygenation of other isomers to the
corresponding aromatic aldehydes. Photooxygenation of a ring-
subsitituted toluene with an electron-withdrawing substituent
(p-tolunitrile) by molecular oxygen is also made possible by
use of tetrafluoro-p-dicyanobenzene as the photocatalyst for the
photooxygenation of p-tolunitrile. Thus, selective photooxy-
genations of ring-substituted toluenes are finely controlled by
choosing an appropriate photocatalyst with different one-electron
redox potentials and solvents.
-
1
kq () Kqτ ) by photoinduced electron transfer are determined
from the slopes of the linear Stern-Volmer plots of τ0/τ (τ0 )
2
3
3
7 ns in MeCN) vs the quencher concentration. The kq values
thus determined are listed in Table 4 (see typical data in
Supporting Information, Figure S4).
The 1AcrH+* fluorescence was quenched efficiently by
1
+*
electron transfer from xylenes to AcrH , whereas no quench-
7
-1 -1
ing was observed by p-tolualdehyde (kq , 1 × 10 M s ).
The kq value decreases in the order p-xylene > o-xylene >
m-xylene > o-tolualdehyde > toluene > m-tolualdehyde .
p-tolualdehyde (not observed). This order is consistent with the
order of the monooxygenated and dioxygenated product yields
Acknowledgment. This work was partially supported by a
Grant-in-Aid for Scientific Research Priority Area (11228205)
from the Ministry of Education, Culture, Sports, Science and
Technology, Japan.
(
53) Nelsen, S. F.; Kapp, D. L.; Akaba, R.; Evans, D. H. J. Am. Chem. Soc.
1
986, 108, 6863.
(54) Correa, P. E.; Hardy, G.; Riley, D. P. J. Org. Chem. 1988, 53, 1695.
55) Clennan, E. L.; Simmons, W.; Almgren, C. W. J. Am. Chem. Soc. 1981,
(
1
03, 2098.
12858 J. AM. CHEM. SOC.
9
VOL. 125, NO. 42, 2003

Selective Oxygenation of Ring-Substituted Toluenes
A R T I C L E S
Supporting Information Available: Five figures showing
Stern-Volmer plot for the fluorescence quenching of various
photosensitizers by p-tolunitrile, transient absorption spectra of
for the fluorescence quenching of AcrPh+ by ring-substituted
toluenes, and irradiation time dependence of the yield of
p-tolualdehyde (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
1
+*
the photoinduced electron transfer from p-xylene to AcrPh
•
in CHCl3, decay time profile of AcrPh , Stern-Volmer plots
JA036645R
J. AM. CHEM. SOC.
9
VOL. 125, NO. 42, 2003 12859